14 



reflected light with the naked eye or under the microscope ex- 

 hibits the opalescence in some of the irregular patches bounded 

 by cracks, while other areas are dark. In transmitted light 

 under the microscope the areas which showed opalescence 

 exhibit a very faint but quite distinct yellowish tinge, while 

 the other areas are quite colourless. No connection could be 

 traced between the optical orientation of any fragment and 

 the presence or absence of opalescence in it. 



The quartz of the ground-mass is thoroughly allotio- 

 morphic and is clear and free from cracks, but is not 

 otherwise noteworthy. 



Flagioclose. — This is in subidiomorphic crystals and 

 irregular grains up to about 6 mm. in length, strongly zoned 

 and twinned on the albite and pericline laws. Optical measure- 

 ments were hard to get, but extinctions on a section parallel 

 to (010) indicate a composition ranging between andesine 

 (Abgo) and oligoclase (Abg^). In other crystals the outer 

 margins are as acid as Abg^. The composition does not vary 

 continuously, but oscillates somewhat, in some crystals at 

 all events. The felspar is spangled with secondary mica which 

 develops mainly along cleavage planes. There are a few small 

 grains of plagioclase in the ground mass, having the composi- 

 tion Ab^(,, or in other words, the same as the outer zones of 

 the larger crystals. The latter dovetail into the minerals of 

 the ground-mass, and the inference is that, in the plagioclase 

 as in the microcline, after the crystallization of the pheno- 

 crysts some of the felspar of the residual magma crystallized 

 in optical continuity with the phenocrysts, while the rest went 

 to form separate crystals. 



Biotite. — Apart from the idiomorphic inclusions in 

 microcline this mineral forms, with the second generation of 

 quartz and felspar, a filling between the larger crystals. It 

 varies considerably in size, the largest individuals being about 

 3 mm. in length, while the smallest are mere specks. The 

 former may possibly belong to an earlier generation ; they 

 are very ragged and corroded and contain fairly abundant 

 inclusions of apatite, ilmenite rods, and zircon. 



Much of the smaller biotite is fairly idiomorphic, but 

 quite irregular flakes are found intergrown with and wrapping 

 round the quartz and felspar of the ground-mass ; this biotite 

 is much less marked by inclusions than the larger corroded 

 material. 



Other' Mincrah. — A little muscovite is intergrown with 

 the biotite. Apatite in small stoutish prisms, zircon as tiny 

 crystals in biotite surrounded by dense pleochroic haloes, 

 ilmenite always included in biotite, and a little pyrites com- 

 plete the mineral constitution. 



